Lava is the very hot molten material that flows out of the earth through
volcanoes.

When it cools it becomes rigid and forms volcanic rock. Lava is a mixture of
many different minerals, which crystallise as the lava cools. Sometimes you
can see these crystals glittering in the cooled rock.

There are several different sorts of lava depending upon the type of minerals
dissolved within them. Some lava is exceptionally runny and flows like warm
treacle. Basalt lava is often like this. This lava is runny because the
minerals dissolved in it are poor in a common mineral called silica. Dissolved
gases also help the lava to become runny. Rocks that form from this type of
lava tend to be dark coloured.

At the other end of the scale are lavas that are so thick they can barely
flow at all. This is because they are very rich in the mineral silica, and the
rocks that form on cooling tend to be pale in colour. Rhyolite lava is like
this. This type of lava sometimes never leaves the vent of the volcano, but
plugs it like a cork in a fizzy drink bottle, bottling up the dissolved gases.
Sooner or later, the pressure becomes so enormous that the volcano explodes
in a violent eruption, spewing volcanic dust for great distances around.

In between these two extreme types of lava are ones that are slower moving
and which alternate with ash eruptions.